Electrochemical cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages, including improved safety features. Notwithstanding their advantages, the use of certain of the solid state batteries over repeated charge/discharge cycles may be substantially impaired when they exhibit significant drops in their charge and discharge capacity over repeated cycles, as compared to their initial charge and discharge capacity, due to the decomposition of the solid electrolyte. Specifically, solid batteries employ a solid electrolyte interposed between a cathode and a anode. The solid electrolyte contains either inorganic or an organic matrix as well as a suitable inorganic ion salt. The inorganic matrix may be nonpolymeric or polymeric, whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic constituents include, by way of example, polyethylene oxide, polypropylene oxide, polyethylene imine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula, CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or a lower alkyl of from 1 to 6 carbon atoms.
Electrochemical cells containing a solid electrolyte, i.e., a polymeric matrix, may suffer from low ion conductivity and accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to 250 .mu.m. It is apparent, the reduced thickness of the film, reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance. However, the solid electrolyte also may contain a solvent (plasticizer) which is typically added to the matrix in order to enhance the solubility of an inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrochemical cell.
Suitable solvents, well-known in the art for use in such solvent electrolytes, include by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, and the like.
In a method of forming the solid electrolyte, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques such as heat, ultraviolet radiation, electron beams, etc. so as to form the solid, solvent-containing electrolyte.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid electrochemical cell.
A battery may exhibit a decline in capacity over its lifetime due to decomposition of the inorganic salts present in the solid electrolyte.
For example, LiPF.sub.6 is a typically preferred inorganic salt finding use in solid electrolytes of lithium-containing electrochemical cells. LiPF .sub.6 is known to undergo thermal decomposition at temperatures within the operating range of rechargeable lithium batteries. Decomposition products shorten the battery lifetime and interfere with its operation.
The prior art does not contain a satisfactory method for the quantitative determination of LiPF.sub.6 decomposition products in nonaqueous media.
It would be advantageous to develop a sensitive method for detecting and quantifying the decomposition of lithium salts such as LiPF.sub.6 and its decomposition product(s) in non-aqueous electrolytes. It would then be possible to study methods of inhibiting decomposition in the environment of the electrolyte.
Difficulties are presented in the determination of the decomposition products, of an acid character, in non-aqueous solvents. The use of colored pH indicators in aqueous solutions is well known. However, the concept of pH is not considered relevant in non-aqueous solvents. Consequently, it is not possible to make a direct measurement of acid content in non-aqueous solvent by the use of ordinary water soluble pH indicators.
Furthermore, the classic definition of "acid" is that of a proton donor, and the classic definition of a "base" is that of a proton acceptor. The classic definition of an acid-base indicator is that of a chemical moiety which substantially changes its absorption spectrum in the ultraviolet or visible portion of the spectrum upon acceptance and release of a proton.
The concept of acid and base has been generalized. A generalized acid definition includes Lewis acids and Lewis bases. The Lewis acid is defined as that chemical moiety which accepts an electron pair, and a Lewis base is that chemical moiety which donates an electron pair. Consequently, indicators for non-aqueous Lewis acid-base equilibria are not the same chemical compounds which find use as indicators for hydrogen ion acid-base equilibria in water.